Annular saws have been known for more than a hundred years. In relation to conventional circular saws they have, however, till now had little importance. The fitting of an annular circular saw in the power unit and also the driving of it require considerably more advanced technical solutions than the fitting and driving of a conventional circular saw. It can be said that there are two main principles for how an annular saw blade must be fitted and driven. One principle is based on the fact that one has both drive rollers and support rollers working on the web portion of the saw blade. The drive rollers can either work through gear drive in a cut-out in the saw blade or through friction drive, which is more usual and to be preferred to gears working under hard, wearing conditions. In order that such friction drive shall function properly, the drive rollers must be pressed very strongly against the body of the saw blade. This leads to severe heating of the saw blade so that the latter buckles. Annular saws with drive rollers which work through friction coupling against the body of the saw blade are described for example in U.S. Pat. Nos. 3,930,310 and 4,316,328.
An annular saw according to the second principle with drive against the inner rim of the saw blade is described in U.S. Pat. No. 4,472,880. The annular saw according to this patent is, in relation to earlier proposed designs, an important technical achievement. Through this design for the first time an annular saw has obtained a significant technical application. The design is, however, not completely without disadvantages. In order that a safe friction coupling may occur between the drive surfaces on the two drive pulleys which form part of the drive system and which are pressed in opposite directions towards each other at right angles to the plane of symmetry of the saw blade so that the rim section of the saw blade is gripped between the drive pulleys, the pressing force must be very great. This entails that the rim section is severely heated. Certainly this does not bring about the buckling of the saw blade, expecially as the design permits the saw blade to be cooled with cooling liquid, but it entails a not inconsiderable loss of energy, so that the drive effect is considerably impaired.